Project Summary Cellular membranes not only compartmentalize intracellular processes but also serve as the dynamic hubs for the assembly of many multi-protein signaling complexes, oncoproteins, and tumor suppressors. Accordingly, a majority of current therapeutics (>60%) target membrane proteins that make up approximately 23% of human proteome. In response to environmental cues and pharmacological drugs, the protein complement of cellular membranes is altered to mount a calibrated response, which, if perturbed, impacts the disease state, e.g., cancer, cardiac and neurological disorders. The ubiquitin-proteasome system (UPS) aptly fits the task to swiftly turnover the regulatory proteins with unmatched precision. To date, our understanding of the molecular details of how membrane protein turnover is regulated by UPS-mediated proteolysis remain sketchy. We are conducting the mechanistic studies in the investigations of novel UPS-regulated protein degradation pathways at membranes modeled on our previous work with FBXL2, a highly conserved F-box protein containing a typical C-terminal CaaX prenylation motif for localization to cellular membranes. The integrity of the CaaX motif is necessary for FBXL2 to assemble into an active SCF ubiquitin ligase complex and interact with two substrates, p85? and IP3R3, at cellular membranes. Interestingly, we recently discovered GGtase3, a new mammalian prenyltransferase and identified FBXL2-ubiquitin ligase as the physiological target for prenylation by GGTase3. This proposal uses an interdisciplinary approach to investigate the regulation and biological relevance of membrane anchored protein turnover by UPS in mammalian cell survival and proliferation, and full characterization of GGtase3 biology.